1. Field of the Invention
The invention relates to apparatus for measuring angular displacement, and more particularly to a tilt sensor and controls for remotely displaying angular displacement of a multiplicity of such sensors with minimal power consumption.
2. Description of the Prior Art
There has long been a need for a compact and reliable system for sensing and measuring changes in angular displacement or for establishing and monitoring a reference level, as in construction of high rise buildings, road or bridge stress monitoring and liquid storage tank monitoring in relation to ground movement. It is further desirable, particularly in any system which is intended for use in remote areas with unattended operation, to have very low power consumption. Moreover, the device should have high precision and stability to permit accurate measurements over long periods of time without adjustment. It is desirable that the sensing device provide for a remote monitoring location so that the sensor can most advantageously be located, which location may be in conflict with ready observation. It is further desirable that such apparatus provide for initiating an alarm signal when the measured displacement exceeds some predetermined angle, so as to call attention to the need for adjustment or corrective measures.
One prior art device is the electronic inclination gauge described in U.S. Pat. No. 4,167,818, issued Sept. 18, 1979 and assigned to Robert Cantarella. This device provided a gravity sensing potentiometer operating in a balanced bridge circuit with numerical readout means integrally incorporated. However, use of a direct current exciting circuit leads to errors due to drift and D.C. offset and no provision was made for remote displays of the angular displacement. Another sensor apparatus was described by A. A. Hakhverdin in U.S. Pat. No. 4,377,912, issued Mar. 29, 1983. In that invention, an elongated sealed chamber, partly filled with a liquid dielectric, responded to changes in inclination by producing a corresponding change in the capacitance of electrodes placed therein. When used in association with a tuned-collector tuned-base oscillator, it provided a device sensitive to very small changes in inclination. However, such an LC oscillator is inherently unstable, and unsuitable for precise measurements over long periods of time without frequent recalibration. Moreover, this invention provided neither for remote readout nor a threshold alarm.
A system known as the ELECTROLEVEL, supplied by Tilt Measurement Limited, Hertfordshire, England, used a bubble level electrolytic sensor in combination with electronic signal conditioning units to provide an output voltage proportional to the degree of tilt of the sensing head. However, this system has a linear range limited to .+-.5 degrees and consumes relatively high power, so that battery life is short.
In U.S. Pat. No. 4,378,693, filed Feb. 11, 1981 and issued Apr. 5, 1983 to E. C. Ratcliffe, a deflection measuring system is described which develops positive and negative D.C. voltages which are compared to derive a difference D.C. voltage, the magnitude and sign of which are dependent on the angle and direction, respectively, of tilt. However, this system is excited from the A.C. main supply or an auxiliary oscillator, without provision for stabilizing the excitation signal amplitude. Further, the system relies on a D.C. coupled amplifier for nulling the sensor, which leads to errors due to drift and D.C. offset. If the system is provided with high gain to realize sensitivity to small angular deviations, the inherent circuit instability is magnified, as well as sensitivity to undesired transients and tripping of the auxiliary output at the alarm threshold.
The present invention provides a sensitive and precise level sensor capable of ready calibration and stable for long period of times without recalibration, operated in conjunction with a remote console and power source. Provision is made for numerical readout of a multiplicity of sensors and for providing an alarm when a preset threshold at any of the sensors has been exceeded. By the use of AC coupling employing a CMOS oscillator and low power operational amplifiers with high impedance resistor scaling, power consumption is reduced by an order of magnitude over other reported devices for this application. Linear operation over .+-.45 degrees is obtained.